1. Field of the Invention
This invention relates to cordage for use in telecommunications, and, more particularly, but not exclusively, to a helically wound cord of the type used in the telephone communications industry and which includes a plurality of tinsel conductors, each conductor being covered with an insulative composition comprising a polyether polyester thermoplastic composition tubed thereover, and a plasticized polyvinyl chloride jacket encompassing the insulated conductors.
2. Description of the Prior Art
Cords used on telephone instruments to connect a handset to a base must have sufficient retractility to insure that they will return promptly to their normal retracted form after having been extended and then released. However, such cords which are commonly known as spring cords must not be so strongly retractile that they require an excessive amount of force to extend them. If a spring cord is too unyielding, instead of the cord extending when a pull is exerted thereon, the instrument to which it is attached may be moved on or pulled from its support. Readily extensible spring cords are desirable particularly when the spring cords are connected to lightweight desk-type or bedroom-type telephone handsets. Further, it is economically desirable to obtain an extended length with as short a length of cordage as possible and, from an appearance standpoint, it is desirable that the retracted length of the spring cord be as short as possible.
Spring cords of the type used on telephone instruments are generally constructed of highly flexible cordage having a plurality of individually insulated, mandrelated tinsel conductors. Each of these tinsel conductors is made by wrapping a plurality of thin tinsel ribbons of a Phosphor-bronze material spirally around a multi-filament nylon center core. In the past, the tinsel conductor was covered with a nylon knit and then insulated with a pressure-extruded polyvinyl chloride (PVC) composition. Subsequently, the plurality of individually insulated conductors were jacketed with a PVC composition. This construction was designed to permit the repetitive flexure of the cordage for a relatively large number of times as encountered during normal usage and also to permit the cordage to be wound helically during the formation of the spring cords. See, for example, U.S. Pat. No. 3,037,068 issued May 29, 1962 in the name of H. L. Wessel, incorporated by reference hereinto.
The cordage may be formed into a spring cord in a conventional manner. See, for example, U.S. Pats. No. 2,920,351 and 3,024,497 issued on Jan. 12, 1960 and on Mar. 13, 1962, respectively in the names of E. C. Hardesty and D. L. Myers and copending commonly assigned application Ser. No. 641,003 filed Dec. 15, 1975 and now U.S. Pat. No. 3,988,092, all incorporated by reference hereinto.
This complex construction was also necessary because of stringent service requirements placed on telephone cords. The served tinsel conductor construction provided a high degree of cut-through protection to the PVC insulation during flexing. Suitable conductor tensile strength and crush resistance properties were also obtained with this construction. The resilient, plasticized PVC jacket compound which was pressure-extruded over the insulated conductors provided a protective outer jacket and maintained the position of the individual conductors in a round configuration. The above-described cordage design was determined to be suitable for the production of (1) a flexible straight mounting or line cord which connects the telephone housing to a wall terminal and (2) a retractile cord which connects the handset to the telephone housing.
A new modular concept in telephone cordage design includes the replacement of individual spade-tipped conductors with a modular plug connection. Jacks adapted to receive the plugs are mounted in the telephone housing or base and in a wall terminal thereby permitting easy replacement of either the line or spring cord by a customer or an installer. See, for example, U.S. Pat. Nos. 3,699,498 and 3,761,869 issued Oct. 17, 1972 and Sept. 25, 1973, respectively in the names of E. C. Hardesty, C. L. Krumreich, A. E. Mulberger, Jr. and S. W. Walden, and U.S. Pat. No. 3,860,316 issued Jan. 14, 1975 in the name of E. C. Hardesty, all incorporated by reference hereinto.
Conversion to modularity and its associated plug-terminated cordage necessitated the development of telephone cordage having a smaller cross-section than that used in the past. A cordage design suitable for use with the modular plugs incorporated smaller conductors arranged in a parallel relationship, positioned in a single plane, and encapsulated with a somewhat oval-shaped jacket. In addition to reduced installation costs, the modular cord design offered other potential benefits such as, for example, (1) smaller, lighter weight telephone cords requiring less PVC; (2) in-plant mechanization of cord finishing thereby eliminating manual operations; and (3) replacement of the various color-coded conductors with a single color made possible by the single plane parallel arrangement of conductors for easy identification.
As mentioned hereinabove, the dimensional constraints imposed by the modular plugs and jacks necessitated a reduction in the overall size of both the insulated conductors and jacketed, oval-shaped flat cordage. To reduce the size of the insulated conductor, it was necessary to eliminate the knitted nylon covering over the served tinsel. The elimination of the protective nylon knit made it necessary to develop a tough insulation material which would function as a high strength barrier to the cutting action of the tinsel ribbon, as an electrical insulation over the tinsel conductor, and as the primary component to achieve resiliency in a retractile telephone cord. A plasticized nylon insulation replaced the knitted nylon covering over the served tinsel conductor and the outer PVC insulation material over the knit.
The use of nylon in insulating the tinsel conductors has not been altogether satisfactory. In present day markets, the nylon material is costly. In addition, nylon is a highly hygroscopic material and requires special handling to avoid moisture absorption. Moreover, portions of the tinsel ribbon occasionally protrude outwardly and cause protuberances in the slow-crystallizing nylon insulation. As a result, the nylon insulated conductors must be rewound and passed through a die to eliminate the protuberances. The plasticized nylon also has a tendency to creep under load, thereby diminishing somewhat the effectiveness of the strain relief system in the modular plugs disclosed in U.S. Pats. 3,699,498, 3,761,869 and 3,860,316.
It is desirable to insulate the tinsel conductors with a material which has all the beneficial properties of nylon, yet which is less costly. Ideally, the insulation composition would be one which crystallizes quickly after extrusion so that when it draws down on the conductive ribbons in a tubing extrusion operation, the upstanding metal portions will be compressed by the insulating material.